Monday, April 18, 2011

Milk and Soap

This month's Kitchen Science Unit started on Friday with an experiment using milk and soap. Well, actually, it started with water and food coloring, and built up to the grand finale of milk, food coloring, and soap.

We quasi followed the scientific method.
First, I explained what we were going to do. Each child would get a plate with some water on it. I would put 4 drops of food coloring in the water. Then they would dip a Q-tip in the water.
What do you think will happen?
The children hypothesized: "the colors will mix in the water", "the Q-tip will get the color on it", "nothing will happen"...

Then we did the experiment. Each child had a plate of water with four different color drops of food coloring. They dipped a dry Q-tip in the water. Next, they dipped a Q-tip with liquid soap on it in the water.

Since this was a highly "controlled" experiment, when each child announced their findings, there was a wide variety of response. For some the color started mixing with the water right away; for others the colors didn't move until the soapy Q-tip was placed in the water. But everyone noticed that the colors drew slightly towards the soapy Q-tip.

The plates were cleaned and it was time for the second half of the experiment. Milk was placed on the plates. Together we discussed differences between milk and water. Children said: "It comes from a cow", "Milk is healthier than water","It has vitamins", "It has calcium". I explained that milk also has fat in it. The milk I brought in was whole milk so it had more fat than 2% or skim milk.

With a bit of knowledge from the water part of the experiment, the children made hypothesis about what they thought would happen to the colors in milk:
"the same thing", "I have no idea", "The milk is thicker so the color won't mix as much".

Four drops of color were added to the milk. The colors barely dispersed. The children dipped a dry Q-tip into the milk. There was no change. The soapy Q-tip was dipped into the plate and there were squeals of delight. The color rushed away from the Q-tip like an "explosion". After watching, stirring, and a few spills, we cleaned up the experiment and discussed our observations and conclusions.

What happened?: "The milk turned light purple", "The water turned black but the milk turned purple", "The colors moved a lot when you put the soap in the milk."...

After many excited descriptions, I asked the children if they had ever hear of molecules. Many children had, and they shared their knowledge with the rest of the class. Then I tried to give a simple explanation of what had happened with the soap and the milk. The soap molecules attract the fat molecules in the milk.

Here is a link with the description of the experiment if you want to try it again at home. It might be interesting to see what happens with other liquids.